This invention relates to starch derivatives which form reversible gels, some of which form hot gels.
In many compositions, especially those of food systems, starches are often employed in order to provide a gel texture to the composition. Unless the starch is used in a pregelatinized form, the starchcontaining composition must be cooked to effect gelatinization of the starch granules and then cooled, usually for a period of 12 to 24 hours at room temperature, in order to allow gel formation to occur.
It is well known that starch is composed of two fractions, the molecular arrangement of one being linear and the other being branched. The linear fraction of starch is known as amylose and the branched fraction as amylopectin. Gel formation is attributed to the retrogradation of the amylose portion of starch. After gelatinization and upon cooling, the linear chains become oriented in parallel alignment due to the affinity of the chain hydroxyl groups for one another. The hydroxyl groups form associations through hydrogen bonds and the chains are thus bound together forming aggregates which are insoluble in water. In dilute aqueous systems the retrograded starch will precipitate while concentrated solutions or dispersions of the retrograded starch will form a gel. It is well known that phase transformation of a starch gel to a flowable liquid upon heating and reformation of the gel upon cooling are not often readily achieved. Starch gels which require little or no shear in order to melt upon heating are referred to as being thermoreversible.
When a thickened amylose-containing composition which does not form a gel is desired, a derivatized starch is often employed. By introducing substituent groups along the starch chain to interfere with the retrogradation process, non-gelling starches are obtained which are referred to as being stabilized. Common stabilization modifications may be accomplished by esterifying or etherifying some of the hydroxyl groups along the starch chain.
The following references describe various starch ester and starch ether preparations.
U.S. Pat. No. 2,661,349 (issued on Dec. 1, 1953 to Caldwell et al.) is directed to the preparation of substituted polysaccharides by reacting starch with succinic or glutaric acid anhydrides containing a C.sub.5 -C.sub.18 substituent group to produce starch acid esters.
U.S. Pat. No. 2,876,217 (issued on Mar. 3, 1959 to E. Paschall) is directed to the preparation of cationic starch ethers by treating starch with the reaction product of epihalohydrin and a tertiary amine. The reagent is said to contain alkyl or alkenyl radicals which may comprise up to 18 carbon atoms.
U.S. Pat. No. Re. 28,809 (issued May 11, 1976 to M. Tessler), which is a reissue of U.S. Pat. No. 3,720,663 (issued on Mar. 13, 1973 to M. Tessler), is directed to the preparation of starch esters by reacting starch with an imidazolide of a C.sub.1 -C.sub.20 monocarboxylic or monosulfonic acid. U.S. Pat. No. 4,020,272 (issued on Apr. 26, 1977 to M. Tessler) further describes starch esters prepared by reactions of starch with N,N'-disubstituted imidazolium salts of C.sub.1 -C.sub.20 monocarboxylic or monosulfonic acids.
U.S. Pat. No. 4,387,221 (issued on June 7, 1983 to M. Tessler et al.) is directed to the preparation of C.sub.1 -C.sub.22 alkyl or alkenyl sulfosuccinate starch half esters.
There are many food and industrial systems which would benefit by employing a body and consistency imparting vehicle, i.e., a thickener, which have the ability to form a gel texture rapidly while hot instead of requiring the system to cool substantially before gel formation begins. Systems which would form a gel without any heating would also be useful in food systems which require no cooking.
Moreover, it would be of considerable importance to provide a gel which is easily capable of phase transformation upon cooling or heating in order to render subsequent homogeneous incorporation of solid or dissolved components to the system.
There is therefore a need for gelling starches which rapidly form reversible gels at room temperature without cooking or at relatively high temperatures after cooking. There is also a need for gelling starches which form hot gels after cooking.